1. Field of the Invention
This invention relates to an imager suitably provided on e.g., a video camera for reducing the aliasing noise contained in an image pickup output from a solid-state imaging device.
2. Description of the Related Art
There has hitherto bee known an imager employing a solid-state imaging device such as a charge-coupled device (CCD). Such solid-state imaging device has a number of light receiving elements of small sized areas arranged in a lattice pattern on a light receiving surface of a semiconductor substrate. An imaging optical system reads out imaging charges generated by an incident light spot formed on the light receiving surface at a pre-set time interval and outputs the imaging charges as an imaging output at a pre-set time interval.
The solid-state imaging device, employed in such imager, has a number of light receiving devices, arrayed in a lattice pattern at a pre-set interval, and vertical and horizontal transfer lines for transferring imaging charges by the imaging light received by the light receiving elements, as shown in FIG. 1A. On the surface of each light receiving element, there is arrayed a color filter for limiting the wavelength of the imaging light incident on the light receiving elements.
Such solid-state imaging device is configured for alternately reading out the sum of charges of a reference light-receiving element and a light receiving element disposed directly above the reference element and the sum of charges of the reference light-receiving element and a light receiving element disposed directly below the reference element by way of so-called field readout. Specifically, for an even field, the sum of charges of a pixel above the reference pixel and the charges of the reference pixel is read out to a vertical transfer line, as shown in FIG. 1A. For an odd field, the sum of charges of a pixel below the reference pixel and the charges of the reference pixel is read out to a vertical transfer line, as shown in FIG. 1B. A one-frame picture is composed by an imaging output of consecutive odd and even fields.
By such field readout, the field frequency can be twice the frame frequency for improving the dynamic resolution of an imaging output.
However, with such solid-state imaging device, since the imaging light is sampled by a horizontal spatial sampling frequency fH and a vertical spatial sampling frequency fV, as determined by the array of the light receiving elements, in order to provide an imaging output, the frequency components higher than the spatial sampling frequency of the imaging light are aliased to a frequency area lower than the spatial sampling frequency, thus producing an aliasing noise superimposed on the imaging output.
Of the aliasing noise, the vertical frequency component can be reduced to a certain extent by the above field readout. However, the horizontal frequency component is left as it is.
Therefore, in the conventional imager, a light beam of the imaging light is separated into plural spots by plural double refraction plates, such as quartz plates, while an optical low-pass filter having a trap point corresponding to the zero imaging light intensity in the spatial frequency distribution of the imaging light is used for diminishing the spatial frequency components not less than one-half the spatial sampling frequency for reducing the aliasing noise.
Such low-pass filter has two to four double refraction plates of, for example, quartz plates, layered together and arranged so that the optical axes thereof are disposed at an angle of 45.degree. with respect to the horizontal direction of the light receiving elements of the solid-state imaging device.
Such optical low-pass filter has in its spatial frequency spectrum two trap lines having positive and negative gradients and intersecting the frequency axis fSH at a frequency position (fSH=1, fSV=0) and another trap line intersecting the frequency axis fSH at a frequency position (fSH=1/2, fSV=0) and extending parallel to the frequency axis fSV, as shown in FIG. 2A.
Therefore, the optical filter has trap points on a horizontal modulation transfer function (MTF) characteristic curve at two points on the horizontal MTF curve corresponding to fSH=1/2 and 1, as shown in FIG. 2B, while having a trap point on the vertical MTF curve at a point corresponding to fSV slightly larger than 1, as shown in FIG. 2C.
Another known optical low-pass filter has in its spatial frequency spectrum two trap lines intersecting the frequency axis fSH at frequency positions (fSH=1/2, fSV=0) and (fSH=1, fSV=0) and extending parallel to the frequency axis fSH, while having a trap line intersecting the frequency axis fSV at frequency positions (fSH=0, fSV=1/2+.delta.) and extending parallel to the frequency axis fSH, as shown in FIG. 3A.
The optical filter has trap points on a horizontal modulation transfer function (MTF) characteristic curve at two points on the horizontal MTF curve corresponding to fSH=1/2 and 1, as shown in FIG. 3B, while having a trap point on the vertical MTF curve at a point corresponding to fSV slightly larger than 1/2, as shown in FIG. 3C.
By limiting the spatial frequency components of the imaging light incident on the light receiving elements using the above optical low-pass filter, it becomes possible to reduce the aliasing noise in the CCD imaging output.
For coping with a still picture in a digital still camera, frame readout, capable of accurately grasping an instant, is appropriate. The reason is that, even if the readout time is twice that for field readout, it is required with frame readout to seize a picture of higher resolution. For frame readout, electrical charges of all light receiving elements are read out on the vertical transfer line from frame to frame. However, if the optical low-pass filter as described above is used, suppression of the vertical components of the aliasing noise cannot be achieved, in distinction from the case of using the optical low-pass filter.
Therefore, two trap points are desirably provided on the MTF characteristics in the vertical direction as in the horizontal direction.
If the CCD is designed to cope with a camcorder, imaging charges imaged by light receiving elements are read out from the light receiving elements at a frequency equal to an integer number times the sub-carrier frequency of the NTSC signals of 3.58 MHz. Since the display period of a horizontal line (1H) remains fixed, the number of the light receiving elements in the horizontal direction is determined by the readout frequency. The number of the light receiving elements in the vertical direction corresponds to the number of the scanning lines. Since the interval between the light receiving elements is generally determined by the shape of a display screen and the number of the light receiving elements, the light receiving elements are arrayed in a rectangular pattern, with the spatial sampling frequency in the longitudinal direction differing from that in the transverse direction. If the above-mentioned trap points are provided in association with the spatial sampling frequencies differing in the longitudinal and transverse directions, the optical low-pass filter tends to be complex in structure.